Low alloy high strength seamless steel pipe for oil country tubular goods

ABSTRACT

The steel pipe of the present invention is a low alloy high strength seamless steel pipe for oil country tubular goods including a composition containing, in terms of mass %, C: 0.23 to 0.27%, Si: 0.01 to 0.35%, Mn: 0.45 to 0.70%, P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, Al: 0.015 to 0.080%, Cu: 0.02 to 0.09%, Cr: 0.8 to 1.5%, Mo: 0.5 to 1.0%, Nb: 0.02 to 0.05%, B: 0.0015 to 0.0030%, Ti: 0.005 to 0.020%, and N: 0.005% or less, and having a ratio of the Ti content to the N content (Ti/N) of 3.0 to 4.0, with the balance being Fe and inevitable impurities, the steel pipe having a ratio of a stress at a strain of 0.7% to a stress at a strain of 0.4% in a stress-strain curve of 1.02 or less and a yield strength of 655 MPa or more.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2016/004914, filed Nov. 18, 2016, which claims priority to Japanese Patent Application No. 2016-036574, filed Feb. 29, 2016, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a high strength seamless steel pipe for oil country tubular goods or gas well, which is excellent in sulfide stress corrosion cracking resistance (SSC resistance) especially in a hydrogen sulfide-containing sour environment. The term “high strength” referred to herein refers to a case of having a strength of T95 grade or more according to the API Standards, namely a strength of 655 MPa or more (95 ksi or more) in terms of yield strength.

BACKGROUND OF THE INVENTION

In recent years, from the viewpoints of a substantial increase in prices of crude oil and expected drying up of oil resources in the near future, the development of a high-depth oil field which has hitherto been disregarded, or an oil field or gas field, etc. in a severe corrosive environment that is a so-called sour environment containing hydrogen sulfide, etc. is eagerly performed. Steel pipes for oil country tubular goods which are used in such an environment are required to have such a material quality that they have both high strength and excellent corrosion resistance (sour resistance).

In response to such a requirement, for example, PTL 1 discloses a steel for oil country tubular goods having excellent sulfide stress corrosion cracking resistance, which is composed of a low alloy steel containing C: 0.2 to 0.35%, Cr: 0.2 to 0.7%, Mo: 0.1 to 0.5%, and V: 0.1 to 0.3% in terms of weight %, and in which the total amount of precipitated carbides and the proportion of an MC type carbide thereamong are prescribed.

In addition, PTL 2 discloses a steel material for oil country tubular goods having excellent sulfide stress corrosion cracking resistance, which contains C: 0.15 to 0.30%, Si: 0.05 to 1.0%, Mn: 0.10 to 1.0%, P: 0.025% or less, S: 0.005% or less, Cr: 0.1 to 1.5%, Mo: 0.1 to 1.0%, Al: 0.003 to 0.08%, N: 0.008% or less, B: 0.0005 to 0.010%, and Ca+O (oxygen): 0.008% or less in terms of mass %, and further contains one or more selected from Ti: 0.005 to 0.05%, Nb: 0.05% or less, Zr: 0.05% or less, and V: 0.30% or less, and in which with respect to properties of inclusions in steel, a maximum length of continuous non-metallic inclusions and the number of grains having a diameter of 20 μm or more are prescribed.

In addition, PTL 3 discloses a steel for oil country tubular goods having excellent sulfide stress corrosion cracking resistance, which contains C: 0.15 to 0.35%, Si: 0.1 to 1.5%, Mn: 0.1 to 2.5%, P: 0.025% or less, S: 0.004% or less, sol.Al: 0.001 to 0.1%, and Ca: 0.0005 to 0.005% in terms of mass %, and in which a Ca-based non-metallic inclusion composition and a composite oxide of Ca and Al are prescribed, and the hardness of the steel is prescribed by HRC.

CITATION LIST Patent Literature

PTL 1: JP-A-2000-178682

PTL 2: JP-A-2001-172739

PTL 3: JP-A-2002-60893

SUMMARY OF THE INVENTION

The sulfide stress corrosion cracking resistance of steel as referred to in the technologies disclosed in these PTLs 1 to 3 means the presence or absence of the generation of SSC when immersing a round bar tensile specimen in a test bath described in NACE (an abbreviation of National Association of Corrosion Engineering) TM0177 for 720 hours while loading a specified stress according to the NACE TM0177 method A. On the other hand, in recent years, for the purpose of securing more safety of steel pipes for oil country tubular goods, a stress intensity factor K_(ISSC) value in a hydrogen sulfide-containing sour environment obtained by carrying out the DCB (double cantilever beam) test as prescribed according to the NACE TM0177 method D is being demanded to satisfy a prescribed value or more. The above-described prior art does not disclose a specific countermeasure for enhancing such a K_(ISSC) value.

In view of the foregoing problem, aspects of the present invention have been made, and an object of these aspects of the present invention is to provide a low alloy high strength seamless steel pipe for oil country tubular goods, which has excellent sulfide stress corrosion cracking resistance (SSC resistance) in a hydrogen sulfide-containing sour environment while having a high strength of T95 grade or more according to the API Standards, and specifically, stably shows a high K_(ISSC) value.

In order to solve the foregoing problem, the present inventors first collected every three or more DCB specimens having a thickness of 10 mm, a width of 25 mm, and a length of 100 mm from seamless steel pipes having various chemical compositions and micro structures of steel and having a yield strength of 655 MPa or more on the basis of the NACE TM0177 method D and provided for a DCB test. As a test bath of the DCB test, an aqueous solution containing 5 mass % of NaCl and 0.5 mass % of CH₃COOH of 24° C. and saturated with a hydrogen sulfide gas of 1 atm (0.1 MPa) was used. The DCB specimens into which a wedge had been introduced under a predetermined condition were immersed in this test bath for 336 hours, a length of a crack generated in the DCB specimens during the immersion and a lift-off load P were then measured, and K_(ISSC) (MPa√m) was calculated according to the following equation (2). K _(ISSC) ={Pa(2√3+2.38h/a)(B/B _(n))^(1/√3) }/Bh ^(3/2)  (2)

Here, FIG. 1 is a schematic view of a DCB specimen. As shown in FIG. 1, h is a height of each arm of the DCB specimen; B is a thickness of the DCB specimen; and B_(n) is a web thickness of the DCB specimen. For these, numerical values prescribed in the NACE TM0177 method D were used. A target of the K_(ISSC) value was set to 26.4 MPa√m or more (24 ksi√inch or more) from a supposed maximum notch defect of oil country tubular goods and applied load condition. A graph resulting from sorting the obtained K_(ISSC) values with an average hardness (Rockwell C scale hardness) of the seamless steel pipe provided with a specimen is shown in FIG. 2. It was noted that though the K_(ISSC) values obtained by the DCB test tend to decrease with an increase of the hardness of the seamless steel pipe, the numerical values are largely scattered even at the same hardness.

As a result of extensive and intensive investigations regarding a cause of this scattering, it was determined that the scattering varies with the kind of steel pipe, and a degree of the scattering is different depending upon a stress-strain curve obtained when measuring the yield strength of steel pipe. FIG. 3 shows examples of the stress-strain curve. In the two stress-strain curves of steel pipe (a solid line A and a broken line B) shown in FIG. 3, though the stress values at a strain of 0.5 to 0.7% corresponding to the yield stress do not vary, one of them (broken line B) reveals continuous yielding, whereas the other (solid line A) reveals an upper yield point. Then, it was found that in the steel revealing the stress-strain curve (broken line B) of continuous yielding type, the scattering in the K_(ISSC) value is large. The present inventors further made extensive and intensive investigations and sorted the dimensions of the scattering in the K_(ISSC) value by (σ_(0.7)/σ_(0.4)) of this stress-strain curve. As a result, it was found that as shown in FIG. 4, by regulating the (σ_(0.7)/σ_(0.4)) of seamless steel pipe to 1.02 or less, the scattering in the K_(ISSC) value can be reduced to approximately half as compared with the case where the (σ_(0.7)/σ_(0.4)) is more than 1.02.

What the scattering in the K_(ISSC) value is reduced to approximately half means that in a hardness-K_(ISSC) value correlation, the hardness of steel as a lower limit of the scattering in the K_(ISSC) value extends to the high hardness side. Specifically, in FIG. 4, in the case where the (σ_(0.7)/σ_(0.4)) of the steel pipe exceeds 1.02 (see white circles in the drawing), even when the Rockwell C scale hardness is 24.3, values lower than 26.4 MPa√m as a target K_(ISSC) value are generated, whereas in the case where the (σ_(0.7)/σ_(0.4)) of the steel pipe is 1.02 or less (see black circles in the drawing), even when the Rockwell C scale hardness is a high value as 27.0, 26.4 MPa√m may be satisfied. That is, even when highly strengthened, a high K_(ISSC) value can be stably obtained.

In the light of the above, there was obtained such a finding that a high K_(ISSC) value can be stably obtained while highly strengthening a seamless steel pipe to be used in a hydrogen sulfide-containing sour environment. As for the reason why when a value of the ratio of the stress (σ_(0.7)) at a strain of 0.7% to the stress (σ_(0.4)) at a strain of 0.4% in the stress-strain curve of seamless steel pipe is low, a high K_(ISSC) value can be stably obtained, the following reason may be thought. That is, when a stress is given in a state where an initial notch is present as in the DCB test, there is a possibility that plastic deformation is caused at an end of the notch, and in the case where plastic deformation is caused, the sensitivity to sulfide stress corrosion cracking increases. On the other hand, as shown in FIG. 3, when the (σ_(0.7)/σ_(0.4)) is high, namely in a strain region of 0.4 to 0.7%, in the case of a steel having such tensile properties that continuous yielding is not yet revealed (solid line A), plastic deformation of a notched end can be inhibited. Thus, the sensitivity to sulfide stress corrosion cracking does not change, and a high K_(ISSC) value is stably obtained.

In order to stably regulate the (σ_(0.7)/σ_(0.4)) of seamless steel pipe to 1.02 or less, in addition to limitation of a chemical composition of steel as described later, it is required to regulate a micro structure to martensite such that the stress-strain curve is not made a continuous yielding type, to suppress the formation of a micro structure other than martensite as far as possible, and further to increase a quenching temperature during quenching to solid-solve Mo as far as possible for the purpose of increasing a secondary precipitation amount of Mo. With respect to the above-described secondary precipitation amount, precipitated Mo having been precipitated before quenching is defined as a primary precipitate, and precipitated Mo that is solid-solved during quenching and precipitated after tempering is defined as a secondary precipitate.

Meanwhile, in order to increase the σ_(0.4) value, it is required to subject the crystal grains to grain refining, and conversely, the quenching temperature is preferably lower. In order to make the both compatible with each other, in producing a seamless steel pipe, first, the rolling finishing temperature of hot rolling for forming a steel pipe is increased, and after finishing of rolling, direct quenching (also referred to as “DQ”; DQ refers to the matter that at the finishing stage of hot rolling, quenching is immediately performed from a state where the steel pipe temperature is still high) is applied. That is, when the rolling finishing temperature is increased to once solid-solve Mo as far as possible, and thereafter, the quenching temperature during quenching and tempering heat treatment of the steel pipe is lowered, both the increase of the above-described secondary precipitation amount of Mo and the grain refining of the micro structure are made compatible with each other, whereby the (σ_(0.7)/σ_(0.4)) can be stably regulated to 1.02 or less. In addition, after hot rolling of steel pipe, in the case where DQ is not applicable, by performing the quenching and tempering heat treatment plural times, in particular, by making the initial quenching temperature high as 1,000° C. or higher, the effect of DQ can be substituted.

Aspects of the present invention have been accomplished on the basis of such findings and has the following gist.

[1] A low alloy high strength seamless steel pipe for oil country tubular goods comprising a composition containing, in terms of mass %,

C: 0.23 to 0.27%,

Si: 0.01 to 0.35%,

Mn: 0.45 to 0.70%,

P: 0.010% or less,

S: 0.001% or less,

O: 0.0015% or less,

Al: 0.015 to 0.080%,

Cu: 0.02 to 0.09%,

Cr: 0.8 to 1.5%,

Mo: 0.5 to 1.0%,

Nb: 0.02 to 0.05%,

B: 0.0015 to 0.0030%,

Ti: 0.005 to 0.020%, and

N: 0.005% or less,

and having a value of a ratio of the Ti content to the N content (Ti/N) of 3.0 to 4.0,

with the balance being Fe and inevitable impurities,

the steel pipe having a value (σ_(0.7)/σ_(0.4)), as a ratio of a stress at a strain of 0.7% to a stress at a strain of 0.4% in a stress-strain curve, of 1.02 or less and a yield strength of 655 MPa or more.

[2] The low alloy high strength seamless steel pipe for oil country tubular goods as set forth in the item [1], which further contains, in addition to the composition, one or more selected from, in terms of mass %,

V: 0.01 to 0.06%,

W: 0.1 to 0.2%, and

Zr: 0.005 to 0.03%.

[3] The low alloy high strength seamless steel pipe for oil country tubular goods as set forth in the item [1] or [2], which further contains, in addition to the composition, in terms of mass %,

Ca: 0.0005 to 0.0030%,

and has the number of oxide-based non-metallic inclusions in steel comprised of Ca and Al and having a major diameter of 5 μm or more, whose composition ratio satisfies, in terms of mass %, the following equation (1), of 20 or less per 100 mm²: (CaO)/(Al₂O₃)≥4.0  (1)

The term “high strength” as referred to herein refers to a strength of T95 grade or more according to the API Standards, namely a strength of 655 MPa or more (95 ksi or more) in terms of yield strength. Although an upper limit value of the yield strength is not particularly limited, it is preferably 825 MPa.

The low alloy high strength seamless steel pipe for oil country tubular goods according to aspects of the present invention is excellent in sulfide stress corrosion cracking resistance (SSC resistance). What the sulfide stress corrosion cracking resistance is excellent refers to the matter that when a DCB test using, as a test bath, an aqueous solution containing 5 mass % of NaCl and 0.5 mass % of CH₃COOH of 24° C. and saturated with a hydrogen sulfide gas of 1 atm (0.1 MPa), that is a DCB test according to the NACE TM0177 method D, is performed three times, K_(ISSC) obtained according to the above-described equation (2) is stably 26.4 MPa√m or more in all of the three-times test.

, In accordance with aspects of the present invention, it is possible to provide a low alloy high strength seamless steel pipe for oil country tubular goods having excellent sulfide stress corrosion cracking resistance (SSC resistance) in a hydrogen sulfide-containing sour environment, and specially, exhibiting stably a high K_(ISSC) value, while having a high strength of T95 grade or more according to the API Standards.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a DCB specimen.

FIG. 2 is a graph showing a relation between hardness and K_(ISSC) value of a steel pipe.

FIG. 3 is a graph showing a stress-strain curve of steel pipes having a different scattering in the K_(ISSC) value.

FIG. 4 is a graph showing the matter that by regulating (σ_(0.7)/σ_(0.4)) obtained from the stress-strain curve of steel pipe to 1.02 or less, a scattering in the K_(ISSC) value decreases.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The steel pipe according to aspects of the present invention is a low alloy high strength seamless steel pipe for oil country tubular goods comprising a composition containing, in terms of mass %, C: 0.23 to 0.27%, Si: 0.01 to 0.35%, Mn: 0.45 to 0.70%, P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, Al: 0.015 to 0.080%, Cu: 0.02 to 0.09%, Cr: 0.8 to 1.5%, Mo: 0.5 to 1.0%, Nb: 0.02 to 0.05%, B: 0.0015 to 0.0030%, Ti: 0.005 to 0.020%, and N: 0.005% or less, and having a value of a ratio of the Ti content to the N content (Ti/N) of 3.0 to 4.0, with the balance being Fe and inevitable impurities, the steel pipe having a value (σ_(0.7)/σ_(0.4)), as a ratio of a stress at a strain of 0.7% to a stress at a strain of 0.4% in a stress-strain curve, of 1.02 or less and a yield strength of 655 MPa or more.

First of all, the reason for limiting the chemical composition of the steel pipe according to aspects of the present invention is described. The term “mass %” is hereinafter referred to simply as “%” unless otherwise indicated.

C: 0.23 to 0.27%

C has a function of increasing the strength of steel and is an important element for securing the desired strength. In order to realize high strengthening to such an extent that the yield strength is 655 MPa or more, it is required to contain C of 0.23% or more. On the other hand, when the content of C exceeds 0.27%, a remarkable increase of (σ_(0.7)/σ_(0.4)) as described later is caused, and a scattering in the K_(ISSC) value becomes large. For this reason, the content of C is limited to 0.23 to 0.27%, and preferably 0.24% or more.

Si: 0.01 to 0.35%

Si is an element functioning as a deoxidizer and having a function of increasing the strength of steel upon being solid-solved in steel and suppressing rapid softening at the time of tempering. In order to obtain such an effect, it is required to contain Si of 0.01% or more. On the other hand, when the content of Si exceeds 0.35%, coarse oxide-based inclusions are formed, and a scattering in the K_(ISSC) value becomes large. For this reason, the content of Si is limited to 0.01 to 0.35%, and preferably 0.01 to 0.04%.

Mn: 0.45 to 0.70%

Mn is an element having a function of increasing the strength of steel through an improvement in quenching hardenability and of preventing grain boundary embrittlement to be caused due to S by bonding to S and fixing S as MnS. In accordance with aspects of the present invention, it is required to contain Mn of 0.45% or more. On the other hand, when the content of Mn exceeds 0.70%, a remarkable increase of (σ_(0.7)/σ_(0.4)) is caused, and a scattering in the K_(ISSC) value becomes large. For this reason, the content of Mn is limited to 0.45 to 0.70%. The content of Mn is preferably 0.50% or more, and preferably 0.65% or less.

P: 0.010% or Less

P shows a tendency to segregate in grain boundaries or the like in a solid-solution state and to cause grain boundary embrittlement cracking or the like, and is thus desirably decreased in amount as far as possible. However, the content of up to 0.010% is permissible. Thus, the content of P is limited to 0.010% or less.

S: 0.001% or Less

S is mostly present as sulfide-based inclusions in steel and deteriorates ductility, toughness, and corrosion resistance, such as sulfide stress corrosion cracking resistance, etc. There is a case where S is partially present in a solid-solution state; in this case, however, S shows a tendency to segregate in grain boundaries or the like and to cause grain boundary embrittlement cracking or the like. Thus, it is desired to decrease the amount of S as far as possible. However, an excessive decrease in amount rapidly increases smelting costs. Thus, in accordance with aspects of the present invention, the content of S is limited to 0.001% or less at which adverse effects are permissible.

O (Oxygen): 0.0015% or Less

O (oxygen) is an inevitable impurity and is present as oxides of Al, Si, and so on in the steel. In particular, when the number of coarse oxides thereof is large, a scattering in the K_(ISSC) value is caused to become large. For this reason, the content of O (oxygen) is limited to 0.0015% or less at which adverse effects are permissible. The content of O (oxygen) is preferably 0.0010% or less.

Al: 0.015 to 0.080%

Al functions as a deoxidizer and contributes to a decrease of solid-solved N by bonding to N to form AlN. In order to obtain such an effect, it is required to contain Al of 0.015% or more. On the other hand, when the content of Al exceeds 0.080%, oxide-based inclusions increase, thereby making a scattering in the K_(ISSC) value large. For this reason, the content of Al is limited to 0.015 to 0.080%. The content of Al is preferably 0.05% or more, and preferably 0.07% or less.

Cu: 0.02 to 0.09%

Cu is an element having a function of improving the corrosion resistance, and when a minute amount thereof is added, a dense corrosion product is formed; the formation and growth of pits serving as a starting point of SSC are suppressed, and the sulfide stress corrosion cracking resistance is remarkably improved. Thus, in accordance with aspects of the present invention, it is required to contain Cu of 0.02% or more. On the other hand, when the content of Cu exceeds 0.09%, the hot workability during a production process of seamless steel pipe is deteriorated. For this reason, the content of Cu is limited to 0.02 to 0.09%. The content of Cu is preferably 0.03% or more, and preferably 0.05% or less.

Cr: 0.8 to 1.5%

Cr is an element which contributes to an increase in the strength of steel through an improvement in quenching hardenability and improves the corrosion resistance. In addition, Cr bonds to C to form carbides, such as M₃C-based, M₇C₃-based, and M₂₃C₆-based carbides, etc., during tempering. In particular, the M₃C-based carbide improves the resistance of softening by tempering of steel, decreases a change in strength to be caused due to tempering, and contributes to an improvement of the yield strength. In order to achieve the yield strength of 655 MPa or more, it is required to contain Cr of 0.8% or more. On the other hand, even when the content of Cr exceeds 1.5%, the effect is saturated, so that such is economically disadvantageous. For this reason, the content of Cr is limited to 0.8 to 1.5%. The content of Cr is preferably 0.9% or more, and preferably 1.1% or less.

Mo: 0.5 to 1.0%

Mo is an element which contributes to an increase in the strength of steel through an improvement in quenching hardenability and improves the corrosion resistance. With respect to this Mo, the present inventors paid attention especially to a point of forming an M₂C-based carbide. Then, the present inventors have found that the M₂C-based carbide to secondarily have precipitated after tempering improves the resistance of softening by tempering of steel, decreases a change in strength to be caused due to tempering, contributes to an improvement of the yield strength, and converts the shape of stress-strain curve of steel from a continuous yielding type to a yielding type. In this way, by converting the shape of stress-strain curve of steel from a continuous yielding type to a yielding type, an effect for improving the strain is obtained. In order to obtain such an effect, it is required to contain Mo of 0.5% or more. On the other hand, when the content of Mo exceeds 1.0%, the Mo₂C-based carbide becomes coarse and serves as a starting point of the sulfide stress corrosion cracking, thereby rather causing a decrease of the K_(ISSC) value. For this reason, the content of Mo is limited to 0.5 to 1.0%. The content of Mo is preferably 0.55% or more, and preferably 0.75% or less.

Nb: 0.02 to 0.05%

Nb is an element which delays recrystallization in an austenite (γ) temperature region to contribute to refining of γ grains, significantly functions in refining of a lower substructure (for example, a packet, a block, or a lath) at the time of finishing of quenching of steel, and has a function of forming a carbide to strengthen the steel. In order to obtain such an effect, it is required to contain Nb of 0.02% or more. On the other hand, when the content of Nb exceeds 0.05%, precipitation of a coarse precipitate (NbN) is accelerated, resulting in deterioration in the sulfide stress corrosion cracking resistance. For this reason, the content of Nb is limited to 0.02 to 0.05%. The content of Nb is preferably 0.025% or more, and preferably 0.035% or less. The packet as referred to herein is defined as a region composed of a group of laths arranged in parallel and having the same crystal habit plane, and the block is composed of a group of parallel laths having the same orientation.

B: 0.0015 to 0.0030%

B is an element which contributes to an improvement in quenching hardenability at a slight content, and in accordance with aspects of the present invention, it is required to contain B of 0.0015% or more. On the other hand, even when the content of B exceeds 0.0030%, the effect is saturated, or conversely, a desired effect cannot be expected due to the formation of a Fe boride (Fe—B), so that such is economically disadvantageous. For this reason, the content of Bis limited to 0.0015 to 0.0030%. The content of B is preferably 0.0020% to 0.0030%.

Ti: 0.005 to 0.020%

Ti forms a nitride and decreases excessive N in the steel, thereby making the above-described effect of B effective. In addition, Ti is an element which contributes to prevention of coarsening to be caused due to a pinning effect of austenite grains during quenching of steel. In order to obtain such an effect, it is required to contain Ti of 0.005% or more. On the other hand, when the content of Ti exceeds 0.020%, the formation of a coarse MC-type nitride (TiN) is accelerated during casting, resulting in rather coarsening of austenite grains during quenching. For this reason, the content of Ti is limited to 0.005 to 0.020%. The content of Ti is preferably 0.008% or more, and preferably 0.015% or less.

N: 0.005% or Less

N is an inevitable impurity in steel and bonds to an element which forms a nitride of Ti, Nb, Al, or the like, to form an MN-type precipitate. Furthermore, excessive N remaining after forming such a nitride also bonds to B to form a BN precipitate. On this occasion, the effect for improving quenching properties due to the addition of B is lost, and therefore, it is preferred that the excessive N is decreased as far possible. The content of N is limited to 0.005% or less.

Ratio of Ti Content to N Content (Ti/N): 3.0 to 4.0

In order that both the pinning effect of austenite grains due to the formation of a TiN nitride by the addition of Ti and the effect for improving quenching hardenability due to the addition of B through prevention of the BN formation due to suppression of excessive N may be made compatible with each other, the Ti/N is prescribed. In the case where the Ti/N is lower than 3.0, the excessive N is generated, and BN is formed, so that the solid-solved B during quenching is insufficient. As a result, the micro structure at the finishing of quenching becomes a multi-phase structure of martensite and bainite, or martensite and ferrite, and the stress-strain curve after tempering such a multi-phase structure becomes a continuous yielding type, whereby the value of (σ_(0.7)/σ_(0.4)) largely increases. On the other hand, in the case where the T/N exceeds 4.0, the pinning effect of austenite grains is deteriorated due to coarsening of TiN, and the required fine grain structure is not obtained. For this reason, the T/N is limited to 3.0 to 4.0.

The balance other than the above-described components is Fe and inevitable impurities. In addition to the above-described basic composition, one or more selected from V: 0.01 to 0.06%, W: 0.1 to 0.2%, and Zr: 0.005 to 0.03% may be selected and contained, if desired. In addition to the above, Ca of 0.0005 to 0.0030% may be contained, and the number of oxide-based non-metallic inclusions in steel comprised of Ca and Al and having a major diameter of 5 μm or more, whose composition ratio satisfies a relation: (CaO)/(Al₂O₃)≥4.0, in terms of mass %, may be 20 or less per 100 mm².

V: 0.01 to 0.06%

V is an element which forms carbide or a nitride and contributes to strengthening of steel. In order to obtain such an effect, it is required to contain V of 0.01% or more. On the other hand, when the content of V exceeds 0.06%, a V-based carbide is coarsened and becomes a starting point of the sulfide stress corrosion cracking, thereby rather causing a decrease of the K_(ISSC) value. For this reason, in the case where V is contained, the content of V is limited to 0.01 to 0.06%.

W: 0.1 to 0.2%

Similar to Mo, W forms carbide to contribute to an increase in strength due to precipitation hardening, and segregates, in a solid solution, in prior-austenite grain boundaries, thereby contributing to an improvement in the sulfide stress corrosion cracking resistance. In order to obtain such an effect, it is desired to contain W of 0.1% or more. However, when the content of W exceeds 0.2%, the resistance of sulfide stress corrosion cracking is deteriorated. For this reason, in the case where W is contained, the content of W is limited to 0.1 to 0.2%.

Zr: 0.005 to 0.03%

Similar to Ti, Zr forms a nitride and is effective for suppressing the growth of austenite grains during quenching due to a pinning effect. In order to obtain the required effect, it is desired to contain Zr of 0.005% or more. On the other hand, even when the content of Zr exceeds 0.03%, the effect is saturated. For this reason, in the case where Zr is contained, the content of Zr is limited to 0.005 to 0.03%.

Ca: 0.0005 to 0.0030%

Ca is effective for preventing nozzle clogging during continuous casting. In order to obtain the required effect, it is desired to contain Ca of 0.0005% or more. On the other hand, Ca forms an oxide-based non-metallic inclusion complexed with Al, and in particular, in the case where the content of Ca exceeds 0.0030%, a large number of coarse non-metallic inclusions are present, thereby deteriorating the sulfide stress corrosion cracking resistance. Specifically, in view of the fact that inclusions in which a composition ratio of the Ca oxide (CaO) to the Al oxide (Al₂O₃) satisfies the equation (1) in terms of mass % especially give adverse effects, it is desired to regulate the number of inclusions having a major diameter of 5 μm or more and satisfying the equation (1) to 20 or less per 100 mm². The number of inclusions can be calculated in the following manner. That is, from an optional one place in the circumferential direction of an end of a steel pipe, a sample for scanning electron microscope (SEM) of a longitudinal orthogonal cross section of the pipe is collected, and with respect to this sample, at least three places of the pipe outer surface, thick-wall center, and inner surface are subjected to SEM observation of inclusions, a chemical composition is analyzed with a characteristic X-ray analyzer annexed to the SEM, and the number of inclusions is calculated from the analysis results. For this reason, in the case where Ca is contained, the content of Ca is limited to 0.0005 to 0.0030%. In addition, in this case, the number of oxide-based non-metallic inclusions in steel comprised of Ca and Al and having a major diameter of 5 μm or more, whose composition ratio satisfies, in terms of mass %, the following equation (1), is limited to 20 or less per 100 mm². The content of Ca is preferably 0.0010% or more, and preferably 0.0016% or less. (CaO)/(Al₂O₃)≥4.0  (1)

The above-described number of inclusions can be controlled by controlling the charged amount of Al during Al-killed treatment to be performed after finishing of decarburization refining and the addition of Ca in an amount in conformity with the analyzed values of Al, O, and Ca in molten steel before the addition of Ca.

In accordance with aspects of the present invention, though it is not particularly needed to limit the production method of a steel pipe raw material having the above-described composition, it is preferred that a molten steel having the above-described composition is refined by a usually known refining method using a converter, an electric furnace, a vacuum melting furnace, or the like and formed into a steel pipe raw material, such as a billet, etc., by a usual method, such as a continuous casting method, an ingot making-blooming method, etc. The steel pipe raw material is formed into a seamless steel pipe by means of hot forming. In the hot forming method, after piercer perforation, the steel pipe raw material is formed in a predetermined thickness by any method of mandrel mill rolling and plug mill rolling, and thereafter, hot rolling is performed until appropriate diameter-reducing rolling. In order to stably regulate the (σ_(0.7)/σ_(0.4)) to 1.02 or less, it is desired to carry out direct quenching (DQ) after hot rolling. Furthermore, it is required to prevent occurrence of the matter that when the micro structure at the finishing of this DQ becomes a multi-phase structure of martensite and bainite, or martensite and ferrite, after the subsequent quenching and tempering heat treatment; the crystal grain diameter of steel and the secondary precipitation amount of Mo or the like become heterogeneous, whereby the value of (σ_(0.7)/σ_(0.4)) exceeds 1.02. For that reason, in order that the commencement of DQ may be performed from an austenite single phase region, the finishing temperature of hot rolling is preferably at 950° C. or higher. On the other hand, the finishing temperature of DQ is preferably 200° C. or lower. After forming the seamless steel pipe, in order to achieve the target yield strength of 655 MPa or more, quenching (Q) and tempering (T) of the steel pipe are carried out. At this time, from the viewpoint of grain refining of crystal grains, the quenching temperature is preferably set to 930° C. or lower. On the other hand, in the case where the quenching temperature is lower than 860° C., solid-solution of No or the like is insufficient, so that the secondary precipitation amount after finishing of the subsequent tempering cannot be secured. For this reason, the quenching temperature is preferably set to 860 to 930° C. In order to avoid re-transformation of austenite, the tempering temperature is required to be an Act temperature or lower; however, when it is lower than 600° C., the secondary precipitation amount of Mo or the like cannot be secured. For this reason, it is preferred to set the tempering temperature to at least 600° C. or higher.

After hot rolling, in the case where DQ is not applicable, by performing quenching and tempering plural times, in particular, by setting the initial quenching temperature to 950° C. or higher, the effect of DQ can be substituted.

Next, the reason for limiting the mechanical properties of the steel pipe according to aspects of the present invention is described.

The value (σ_(0.7)/σ_(0.4)), as a ratio of a stress (σ_(0.7)) at a strain of 0.7% to a stress (σ_(0.4)) at a strain of 0.4% in the stress-strain curve, is 1.02 or less.

As described previously, the scattering in the K_(ISSC) value is largely different according to the shape of the stress-strain curve of steel. The present inventors made extensive and intensive investigations regarding this point. As a result, it has been found that in the case where the value (σ_(0.7)/σ_(0.4)), as a ratio of a stress (σ_(0.7)) at a strain of 0.7% to a stress (σ_(0.4)) at a strain of 0.4% in the stress-strain curve, is 1.02 or less, the scattering in the K_(ISSC) value is reduced to approximately half. For this reason, in accordance with aspects of the present invention, the (σ_(0.7)/σ_(0.4)) is limited to 1.02 or less.

In accordance with aspects of the present invention, the yield strength, the stress (σ_(0.4)) at a strain of 0.4%, and the stress (σ_(0.7)) at a strain of 0.7% can be measured by the tensile test in conformity with JIS 22241.

In addition, though the micro structure according to aspects of the present invention is not particularly limited; so long as the structure is composed of martensite as a major phase, with the balance being one or more structures of ferrite, residual austenite, perlite, bainite, and the like in an area ratio of 5% or less, the object according to aspects of the invention of the present application can be achieved.

Example 1

Aspects of the present invention are hereunder described in more detail by reference to Examples.

A steel of each of compositions shown in Tables 1 and 2 was refined by the converter method and then continuously cast to prepare abloom slab. This bloom slab was formed into a billet having a round cross section by means of hot rolling. Furthermore, this billet was used as a raw material, heated at a billet heating temperature shown in Tables 3 to 6, and then hot-rolled by Mannesmann piercing-plug mill rolling-diameter-reducing process, and rolling was finished at a rolling finishing temperature shown in Tables 3 to 6, thereby forming a seamless steel pipe. The steel pipe was cooled to room temperature (35° C. or lower) by means of direct quenching (DQ) or air cooling (0.1 to 0.5° C./s) and then heat treated under a heat treatment condition of steel pipe shown in Tables 3 to 6 (Q1 temperature: first quenching temperature, T1 temperature: first tempering temperature, Q2 temperature: second quenching temperature, and T2 temperature: second tempering temperature). A tensile specimen and DCB specimens were each taken from an optional one place in the circumferential direction of an end of the pipe at the stage of finishing of final tempering. The three or more DCB specimens were respectively taken from every steel pipes.

Using the collected tensile specimen, yield strength, a stress (σ_(0.4)) at a strain of 0.4%, and a stress (σ_(0.7)) at a strain of 0.7% were measured by performing the tensile test in conformity with JIS 22241.

In addition, using the collected DCB specimens, the DCB test was carried out in conformity with the NACE TM0177 method D. As a test bath of the DCB test, an aqueous solution containing 5 mass % of NaCl and 0.5 mass % of CH₃COOH of 24° C. and saturated with a hydrogen sulfide gas of 1 atm (0.1 MPa) was used. The DCB specimen into which a wedge had been introduced under a predetermined condition was immersed in this test bath for 336 hours, a length a of a crack generated in the DCB specimen during the immersion and a lift-off load P were then measured, and K_(ISSC) (MPa√m) was calculated according to the following equation (2).

In the case where the yield strength was 655 MPa or more, such was judged to be accepted. In addition, in the case where in all of the three DCB specimens, the K_(ISSC) value was 26.4 MPa√m or more, such was judged to be accepted. K _(ISSC) ={Pa(2√3+2.38h/a)(B/B _(n))^(1/√3) }/Bh ^(3/2)  (2)

Here, h is a height of each arm of the DCB specimen; B is a thickness of the DCB specimen; and B_(n) is a web thickness of the DCB specimen. For these, numerical values prescribed in the NACE TM0177 method D were used (see FIG. 1).

TABLE 1 Steel Chemical composition (mass %) No. C Si Mn P S O Al Cu Cr Mo A 0.24 0.02 0.64 0.008 0.0008 0.0010 0.052 0.04 0.98 0.55 B 0.26 0.04 0.48 0.007 0.0007 0.0009 0.068 0.02 1.01 0.75 C 0.25 0.25 0.61 0.009 0.0009 0.0012 0.054 0.04 0.99 0.58 D 0.27 0.04 0.52 0.001 0.0008 0.0009 0.053 0.03 1.29 0.73 E 0.26 0.03 0.58 0.010 0.0009 0.0010 0.055 0.03 1.04 0.71 F 0.27 0.04 0.52 0.010 0.0009 0.0010 0.061 0.04 1.02 0.60 G 0.25 0.03 0.55 0.009 0.0009 0.0009 0.063 0.03 0.98 0.72 H 0.25 0.04 0.63 0.010 0.0008 0.0008 0.051 0.04 1.11 0.56 I 0.27 0.03 0.54 0.010 0.0010 0.0010 0.033 0.03 0.93 0.55 J 0.27 0.03 0.65 0.010 0.0010 0.0010 0.051 0.04 1.14 0.74 K 0.25 0.02 0.64 0.010 0.0010 0.0010 0.057 0.03 1.09 0.98 L 0.24 0.01 0.52 0.007 0.0010 0.0009 0.066 0.05 0.97 0.73 M 0.25 0.04 0.53 0.010 0.0010 0.0009 0.058 0.04 0.96 0.72 Steel Chemical composition (mass %) No. Nb B Ti N V W Zr Ti/N Division A 0.033 0.0022 0.013 0.0040 — — — 3.3 Compatible example B 0.025 0.0020 0.013 0.0033 — — — 3.9 Compatible example C 0.029 0.0027 0.009 0.0025 — — — 3.6 Compatible example D 0.041 0.0021 0.013 0.0041 — — — 3.2 Compatible example E 0.033 0.0020 0.012 0.0035 — 0.15 — 3.4 Compatible example F 0.028 0.0022 0.012 0.0037 — — 0.015 3.2 Compatible example G 0.031 0.0025 0.011 0.0035 0.027 — — 3.1 Compatible example H 0.033 0.0023 0.010 0.0032 0.042 — — 3.1 Compatible example I 0.035 0.0019 0.015 0.0040 — — — 3.8 Compatible example J 0.024 0.0015 0.012 0.0030 0.050 — — 4.0 Compatible example K 0.044 0.0016 0.014 0.0045 0.041 — — 3.1 Compatible example L 0.029 0.0028 0.009 0.0030 — — — 3.0 Compatible example M 0.031 0.0020 0.012 0.0033 0.033 0.18 0.009 3.6 Compatible example The balance other than the above-described components is Fe and inevitable impurities.

TABLE 2 Steel Chemical composition (mass %) No. C Si Mn P S O Al Cu Cr N 0.22 0.30 0.63 0.010 0.0010 0.0010 0.052 0.04 1.13 O 0.28 0.24 0.46 0.007 0.0009 0.0007 0.041 0.06 0.96 P 0.27 0.33 0.43 0.010 0.0010 0.0010 0.054 0.04 1.09 Q 0.26 0.29 0.76 0.010 0.0009 0.0012 0.044 0.08 0.95 R 0.27 0.34 0.65 0.010 0.0010 0.0010 0.051 0.04 0.66 S 0.27 0.32 0.64 0.010 0.0010 0.0010 0.049 0.04 1.40 T 0.25 0.28 0.47 0.009 0.0008 0.0009 0.036 0.06 0.83 U 0.24 0.02 0.61 0.010 0.0010 0.0008 0.053 0.03 0.98 V 0.26 0.05 0.63 0.009 0.0007 0.0010 0.048 0.08 0.95 W 0.24 0.03 0.63 0.010 0.0010 0.0010 0.057 0.02 0.99 X 0.27 0.33 0.62 0.010 0.0010 0.0010 0.048 0.04 1.35 Y 0.26 0.03 0.51 0.008 0.0007 0.0016 0.056 0.03 0.97 Z 0.27 0.03 0.64 0.010 0.0009 0.0008 0.037 0.04 1.02 AA 0.25 0.03 0.59 0.010 0.0010 0.0010 0.061 0.04 1.00 AB 0.25 0.04 0.51 0.010 0.0009 0.0009 0.055 0.03 0.99 Steel Chemical composition (mass %) No. Mo Nb B Ti N V W Zr Ti/N Division N 0.91 0.041 0.0020 0.012 0.0035 — — — 3.4 Comparison O 0.57 0.025 0.0021 0.013 0.0039 — — — 3.3 Comparison P 0.89 0.042 0.0020 0.013 0.0038 — — — 3.4 Comparison Q 0.56 0.037 0.0017 0.008 0.0025 — — — 3.2 Comparison R 0.98 0.038 0.0022 0.012 0.0031 — — — 3.9 Comparison S 0.40 0.034 0.0027 0.011 0.0028 — — — 3.9 Comparison T 1.15 0.021 0.0016 0.018 0.0047 — — — 3.8 Comparison U 0.55 0.010 0.0019 0.014 0.0045 — — — 3.1 Comparison V 0.61 0.059 0.0019 0.011 0.0034 — — — 3.2 Comparison W 0.54 0.036 0.0020 0.004 0.0013 — — — 3.1 Comparison X 0.97 0.045 0.0011 0.018 0.0045 — — — 4.0 Comparison Y 0.77 0.033 0.0020 0.012 0.0030 — — — 4.0 Comparison Z 0.59 0.027 0.0022 0.018 0.0059 — — — 3.1 Comparison AA 0.73 0.028 0.0020 0.012 0.0042 — — — 2.9 Comparison AB 0.71 0.033 0.0022 0.019 0.0045 — — — 4.2 Comparison The underlined portions fall outside the scope of the present invention. The balance other than the above-described components is Fe and inevitable impurities.

TABLE 3 Steel pipe heat Steel pipe rolling condition treatment condition Steel Pipe Outer Billet Finishing Cooling Q1 T1 pipe Steel thickness diameter heating of rolling after temperature temperature No. No. Ti/N (mm) (mm) (° C.) (° C.) rolling (° C.) (° C.) 1 A 3.3 24.5 177.8 1250 1000 DQ 919 720 2 A 3.3 24.5 177.8 1249 1005 Air 1000 650 cooling 3 B 3.9 24.5 177.8 1255 992 DQ 920 719 4 C 3.6 24.5 177.8 1248 989 DQ 917 704 5 D 3.2 24.5 177.8 1245 1008 DQ 887 688 6 E 3.4 38.1 216.3 1260 1030 DQ 921 708 7 F 3.2 38.1 216.3 1259 1033 DQ 921 710 8 G 3.1 28.9 311.2 1200 1042 DQ 889 692 Steel pipe heat treatment condition Steel Q2 T2 Yield pipe temperature temperature strength K_(ISSC) No. (° C.) (° C.) (MPa) σ_(0.4) σ_(0.7) σ_(0.7)/σ_(0.4) (MPa√m) Remark 1 — — 689 697 690 0.99 30.7 Invention 31.1 32.9 2 920 719 710 697 711 1.02 26.8 Invention 28.9 30.4 3 — — 730 743 728 0.98 29.8 Invention 31.2 32.1 4 — — 693 680 694 1.02 26.4 Invention 28.1 31.8 5 — — 764 757 765 1.01 26.5 Invention 29.1 31.3 6 — 675 691 677 0.98 30.4 Invention 30.8 32.7 7 — — 744 757 742 0.98 29.0 Invention 29.9 31.4 8 — — 779 796 780 0.98 27.4 Invention 29.3 30.0

TABLE 4 Steel pipe heat Steel pipe rolling condition treatment condition Steel Pipe Outer Billet Finishing Cooling Q1 T1 Pipe Steel thickness diameter heating of rolling after temperature temperature No. No. Ti/N (mm) (mm) (° C.) (° C.) rolling (° C.) (° C.) 9 G 3.1 28.9 311.2 1259 1041 Air 1001 599 cooling 10 H 3.1 28.9 311.2 1198 1029 DQ 890 689 11 I 3.8 24.5 311.2 1233 979 DQ 934 729 12 J 4.0 38.1 216.3 1204 1044 DQ 899 651 13 J 4.0 13.8 244.5 1270 1098 Air 1005 599 cooling 14 K 3.1 38.1 216.3 1199 1026 DQ 880 550 15 L 3.0 24.5 177.8 1267 1011 DQ 906 713 16 M 3.6 13.8 244.5 1248 1101 DQ 949 709 Steel pipe heat treatment condition Steel Q2 T2 Yield Pipe temperature temperature strength K_(ISSC) No. (° C.) (° C.) (MPa) σ_(0.4) σ_(0.7) σ_(0.7)/σ_(0.4) (MPa√m) Remark 9 880 688 785 774 782 1.01 26.8 Invention 28.9 30.4 10 — — 794 803 795 0.99 27.0 Invention 27.6 28.9 11 — — 723 710 724 1.02 26.5 Invention 28.9 32.3 12 900 691 781 790 782 0.99 27.2 Invention 28.7 29.5 13 880 692 810 803 811 1.01 26.4 Invention 27.4 28.4 14 880 699 788 782 790 1.01 26.5 Invention 27.2 27.7 15 — — 693 679 693 1.02 26.5 Invention 29.1 31.6 16 — — 825 834 826 0.99 27.1 Invention 28.3 29.3

TABLE 5 Steel pipe heat Steel pipe rolling condition treatment condition Steel Pipe Outer Billet Finishing Cooling Q1 T1 pipe Steel thickness diameter heating of rolling after temperature temperature No. No. Ti/N (mm) (mm) (° C.) (° C.) rolling (° C.) (° C.) 17 N 3.4 24.5 177.8 1255 995 DQ 931 688 18 O 3.3 24.5 177.8 1249 1058 DQ 924 709 19 P 3.4 24.5 177.8 1251 1005 DQ 930 691 20 Q 3.2 24.5 177.8 1256 1037 DQ 922 703 21 R 3.9 24.5 177.8 1249 983 DQ 929 648 22 S 3.9 24.5 177.8 1254 1002 DQ 929 613 23 T 3.8 24.5 177.8 1263 1113 DQ 915 711 24 U 3.1 24.5 177.8 1255 989 DQ 932 695 25 V 3.2 24.5 177.8 1249 992 DQ 927 714 Steel pipe heat treatment condition Steel Q2 T2 Yield pipe temperature temperature strength K_(ISSC) No. (° C.) (° C.) (MPa) σ_(0.4) σ_(0.7) σ_(0.7)/σ_(0.4) (MPa√m) Remark 17 — — 627 628 628 1.00 33.3 Comparison 34.7 35.4 18 — — 956 887 958 1.08 20.2 Comparison 24.2 25.1 19 — — 641 649 643 0.99 32.5 Comparison 33.9 35.1 20 — — 854 785 856 1.09 21.1 Comparison 23.7 26.2 21 — — 648 657 650 0.99 32.9 Comparison 34.5 35.5 22 — — 613 600 612 1.02 31.2 Comparison 32.3 33.7 23 — — 938 861 939 1.02 23.8 Comparison 25.7 26.1 24 — — 677 654 680 1.04 25.5 Comparison 27.7 31.5 25 — — 697 678 698 1.03 22.4 Comparison 25.3 26.2 The underlined portions fall outside the scope of the present invention.

TABLE 6 Steel pipe heat Steel pipe rolling condition treatment condition Steel Pipe Outer Billet Finishing Cooling Q1 T1 pipe Steel thickness diameter heating of rolling after temperature temperature No. No. Ti/N (mm) (mm) (° C.) (° C.) rolling (° C.) (° C.) 26 W 3.1 24.5 177.8 1256 999 DQ 905 705 27 X 4.0 24.5 177.8 1188 966 DQ 902 715 28 Y 4.0 24.5 177.8 1195 1003 DQ 920 718 29 Z 2.9 24.5 177.8 1211 1044 DQ 911 710 30 AA 2.9 28.9 311.2 1258 1009 DQ 899 689 31 AB 4.2 28.9 311.2 1261 1103 DQ 901 690 32 A 3.3 28.9 311.2 1262 1089 DQ 901 589 33 A 3.3 28.9 311.2 1259 1088 Air 1010 600 cooling 34 A 3.3 28.9 311.2 1265 1091 Air 891 689 cooling Steel pipe heat treatment condition Steel Q2 T2 Yield pipe temperature temperature strength K_(ISSC) No. (° C.) (° C.) (MPa) σ_(0.4) σ_(0.7) σ_(0.7)/σ_(0.4) (MPa√m) Remark 26 — — 661 617 660 1.07 24.3 Comparison 25.7 30.9 27 — — 679 648 680 1.05 25.6 Comparison 29.8 32.5 28 — — 721 709 723 1.02 25.2 Comparison 30.7 31.7 29 — — 734 726 733 1.01 25.1 Comparison 26.3 31.1 30 — — 744 703 746 1.06 26.2 Comparison 25.2 30.5 31 — — 761 665 765 1.15 23.2 Comparison 24.9 27.8 32 — — 841 760 844 1.11 24.4 Comparison 25.3 28.9 33 850 689 703 680 707 1.04 23.6 Comparison 27.1 29.9 34 — — 678 653 679 1.04 26.1 Comparison 29.6 32.1 The underlined portions fall outside the scope of the present invention.

In all of the steel pipes 1 to 16 which fall within the scope of the present invention in terms of the chemical composition and (σ_(0.7)/σ_(0.4)), the yield strength was 655 MPa or more, and all of the K_(ISSC) values obtained in the DCB test of every three specimens satisfied the target 26.4 MPa√m or more without causing scattering.

On the other hand, all of Comparative Example 17 (steel No. N) in which the C amount of the chemical composition was lower than the scope of the present invention, Comparative Example 19 (steel No. P) in which the Mn amount was lower than the scope of the present invention, Comparative Example 21 (steel No. R) in which the Cr amount was lower than the scope of the present invention, and Comparative Example 22 (steel No, S) in which the Mo amount was lower than the scope of the present invention could not achieve the yield strength of 655 MPa or more.

In addition, in Comparative Example 18 (steel No. O) in which the C amount of the chemical composition was more than the scope of the present invention and Comparative Example 20 (steel No. Q) in which the Mn amount was more than the scope of the present invention, the (σ_(0.7)/σ_(0.4)) fell outside the scope of the present invention. As a result, all of the three specimens in the DCB test did not satisfy the target 26.4 MPa√m or more.

In addition, in Comparative Example 23 (steel No. T) in which the Mo amount was more than the scope of the present invention, all of the three specimens in the DCB test did not satisfy the target 26.4 MPa√m or more.

In Comparative Example 24 (steel No. U) in which the Nb amount of the chemical composition was lower than the scope of the present invention, the (σ_(0.7)/σ_(0.4)) fell outside the scope of the present invention. As a result, the K_(ISSC) value was largely, scattered, and one of the three pipes in the DCB test did not satisfy the target 26.4 MPa√m or more.

Conversely, in Comparative Example 25 (steel No. V) in which the Nb amount was more than the scope of the present invention, the (σ_(0.7)/σ_(0.4)) fell outside the scope of the present invention. As a result, all of the three specimens in the DCB test did not satisfy the target 26.4 MPa√m or more.

In Comparative Example 26 (steel No. W) in which the Ti amount was lower than the scope of the present invention, the (σ_(0.7)/σ_(0.4)) fell outside the scope of the present invention. As a result, the K_(ISSC) value was largely scattered, and two of the three specimens in the DCB test did not satisfy the target 26.4 MPa√m or more.

In Comparative Example 27 (steel No. X) in which the B amount of the chemical composition was lower than the scope of the present invention, the (σ_(0.7)/σ_(0.4)) fell outside the scope of the present invention. As a result, the K_(ISSC) value was largely scattered, and one of the three specimens in the DCB test did not satisfy the target 26.4 MPa√m or more.

In Comparative Example 28 (steel No. Y) in which the 0 amount of the chemical composition was more than the scope of the present invention and Comparative Example 29 (steel No. Z) in which the N amount was more than the scope of the present invention, the cleanliness was largely deteriorated, so that the K_(ISSC) value was largely scattered, and one or two of the three specimens in the DCB test did not satisfy the target 26.4 MPa√m or more.

In Comparative Example 30 (steel No. AA) in which the Ti/N ratio of the chemical composition was lower than the scope of the present invention, excessive N was present, and therefore, the excessive N was bonded to B during quenching, thereby causing precipitation of BN. As a result, the effective B amount was insufficient, the micro structure immediately after quenching became a composite structure of martensite and bainite, and the (σ_(0.7)/σ_(0.4)) fell outside the scope of the present invention. As a result, the K_(ISSC) value was largely scattered, and two of the three specimens in the DCB test did not satisfy the target 26.4 MPa√m or more.

On the other hand, in Comparative Example 31 (steel No. AB) in which the Ti/N ratio was more than the scope of the present invention, TiN was coarsened so that the sufficient pinning effect was not obtained, the micro structure of steel was coarsened, and the (σ_(0.7)/σ_(0.4)) fell outside the scope of the present invention. As a result, the K_(ISSC) value was largely scattered, and two of the three specimens in the DCB test did not satisfy the target 26.4 MPa√m or more.

In Comparative Examples 32 and 33 in which though the chemical composition was compatible with the scope of the present invention, the final tempering temperature was low, or the quenching temperature before the final tempering was low, the (σ_(0.7)/σ_(0.4)) fell outside the scope of the present invention. As a result, the K_(ISSC) value was largely scattered, and one or two of the three specimens in the DCB test did not satisfy the target 26.4 MPa√m or more. In addition, similarly, in Comparative Example 34 in which the direct quenching (DQ) was not performed, and the quenching and tempering heat treatment of steel pipe was performed only one time, the (σ_(0.7)/σ_(0.4)) fell outside the scope of the present invention. As a result, the K_(ISSC) value was largely scattered, and one of the three specimens in the DCB test did not satisfy the target 26.4 MPa√m or more.

Example 2

A steel of each of compositions shown in Table 7 was refined by the converted method and then continuously cast to prepare a bloom slab. This bloom slab was formed into a billet having a round cross section by means of hot rolling. Furthermore, this billet was used as a raw material, heated at a billet heating temperature shown in Table 8, and then hot-rolled by Mannesmann piercing-plug mill rolling-diameter-reducing process, and rolling was finished at a rolling finishing temperature shown in Table 8, thereby forming a seamless steel pipe. The steel pipe was cooled to room temperature (35° C. or lower) by means of direct quenching (DQ) or air cooling (0.2 to 0.5° C./s) and then heat treated under a heat treatment condition of steel pipe shown in Table 8 (Q1 temperature: first quenching temperature, T1 temperature: first tempering temperature, Q2 temperature: second quenching temperature, and T2 temperature: second tempering temperature). A sample for SEM of a longitudinal orthogonal cross section, a tensile specimen, and DCB specimens were each taken from an optional one place in the circumferential direction of an end of the pipe at the stage of finishing of final tempering. The three or more DCB specimens were respectively taken from every steel pipes.

With respect to the collected sample for SEM, three places of the pipe outer surface, thick-wall center, and inner surface were subjected to SEM observation of inclusions, a chemical composition was analyzed with a characteristic X-ray analyzer annexed to the SEM, and the number (per 100=²) of oxide-based non-metallic inclusions in steel comprised of Ca and Al and having a major diameter of 5 μm or more and satisfying the equation (1) was calculated. (CaO)/(Al₂O₃)≥4.0  (1)

In addition, using the collected tensile specimen, yield strength, a stress (σ_(0.4)) at a strain of 0.4%, and a stress (σ_(0.7)) at a strain of 0.7% were measured by the performing tensile test in conformity with JIS 22241.

In addition, using the collected DCB specimens, the DCB test was carried out in conformity with the NACE TM0177 method D. As a test bath of the DCB test, an aqueous solution containing 5 mass % of NaCl and 0.5 mass % of CH₃COOH of 24° C. and saturated with a hydrogen sulfide gas of 1 atm (0.1 MPa) was used. The DCB specimens into which a wedge had been introduced under a predetermined condition were immersed in this test bath for 336 hours, a length a of a crack generated in the DCB specimens during the immersion and a lift of load P were then measured, and K_(ISSC) (MPa√m) was calculated according to the foregoing equation (2).

In the case where the yield strength was 655 MPa or more, such was judged to be accepted. In addition, in the case where in all of the three DCB specimens, the K_(ISSC) value was 26.4 MPa√m or more, such was judged to be accepted.

TABLE 7 Steel Chemical composition (mass %) No. C Si Mn P S O Al Cu Cr Mo Nb AC 0.25 0.04 0.55 0.008 0.0010 0.0010 0.055 0.03 1.01 0.59 0.025 AD 0.27 0.03 0.64 0.009 0.0009 0.0011 0.061 0.02 0.98 0.71 0.026 AE 0.25 0.04 0.68 0.010 0.0010 0.0014 0.052 0.02 0.86 0.92 0.021 AF 0.26 0.02 0.59 0.010 0.0009 0.0009 0.069 0.04 1.22 0.55 0.023 AG 0.26 0.11 0.62 0.009 0.0009 0.0009 0.051 0.03 0.91 0.89 0.027 AH 0.27 0.06 0.56 0.010 0.0010 0.0012 0.063 0.04 1.41 0.88 0.024 AI 0.26 0.03 0.54 0.009 0.0010 0.0010 0.058 0.04 0.99 0.61 0.024 AJ 0.25 0.09 0.56 0.008 0.0009 0.0009 0.054 0.03 1.00 0.57 0.021 Steel Chemical composition (mass %) No. B Ti N V W Zr Ca Ti/N Division AC 0.0023 0.010 0.0032 — — — 0.0013 3.1 Compatible example AD 0.0026 0.011 0.0035 0.019 — — 0.0016 3.1 Compatible example AE 0.0018 0.008 0.0023 0.021 — 0.022 0.0018 3.5 Compatible example AF 0.0021 0.012 0.0038 — 0.11 0.025 0.0021 3.2 Compatible example AG 0.0017 0.010 0.0033 0.022 0.13 — 0.0015 3.0 Compatible example AH 0.0019 0.013 0.0041 0.039 0.11 0.021 0.0014 3.2 Compatible example AI 0.0019 0.011 0.0035 — — — 0.0034 3.1 Comparison AJ 0.0022 0.012 0.0037 — — — 0.0027 3.2 Compatible example The underlined portions fall outside the scope of the present invention. The balance other than the above-described components is Fe and inevitable impurities.

TABLE 8 Number of Steel pipe heat inclusions Steel pipe rolling condition treatment condition Steel (per 100 Pipe Outer Billet Finishing Cooling Q1 T1 pipe Steel mm²) thickness diameter heating of rolling after temperature temperature No. No. Ti/N (*1) (mm) (mm) (° C.) (° C.) rolling (° C.) (° C.) 2-1 AC 3.1 0 24.5 177.8 1250 1020 DQ 923 710 2-2 AD 3.1 2 28.9 311.2 1211 1033 DQ 861 688 2-3 AE 3.5 9 28.9 311.2 1204 1041 Air 951 705 cooling 2-4 AF 3.2 13  38.1 216.3 1221 1088 DQ 874 699 2-5 AG 3.0 4 24.5 177.8 1248 1001 DQ 878 698 2-6 AH 3.2 3 13.8 244.5 1277 1077 DQ 899 701 2-7 AI 3.1 61  24.5 177.8 1247 1011 DQ 921 708 2-8 AJ 3.2 23  24.5 177.8 1245 1018 DQ 919 711 Steel pipe heat treatment condition Steel Q2 T2 Yield pipe temperature temperature strength K_(ISSC) No. (° C.) (° C.) (MPa) σ_(0.4) σ_(0.7) σ_(0.7)/σ_(0.4) (MPa√m) Remark 2-1 — — 693 705 691 0.98 30.7 Invention 32.1 32.5 2-2 — — 771 782 774 0.99 27.1 Invention 28.4 31.2 2-3 885 693 783 767 782 1.02 26.6 Invention 27.9 30.1 2-4 — — 761 754 762 1.01 26.7 Invention 28.3 30.8 2-5 — — 766 755 763 1.01 26.9 Invention 28.6 31.9 2-6 — — 819 818 818 1.00 27.2 Invention 29.1 31.4 2-7 — — 695 682 696 1.02 20.1 Comparison 26.4 26.9 2-8 — — 691 678 692 1.02 25.3 Comparison 28.9 31.9 The underlined portions fall outside the scope of the present invention. (*1) Number (per 100 mm²) of oxide-based non-metallic inclusions in steel satisfying a relation: (CaO)/(Al₂O₃) ≥ 4.0 and having a major diameter of 5 μm or more.

In all of the steel pipes 2-1 to 2-6 which fall within the scope of the present invention in terms of the chemical composition, the number of inclusions, and (σ_(0.7)/σ_(0.4)), the yield strength was 655 MPa or more, and all of the K_(ISSC) values obtained in the DCB test of every three specimens satisfied the target 26.4 MPa√m or more without causing scattering.

On the other hand, in Comparative Example 2-7 (steel No. AI) in which the upper limit of Ca was more than the scope of the present invention, the K_(ISSC) value was largely scattered, and one of the three specimens in the DCB test did not satisfy the target 26.4 MPa√m or more. In addition, in Comparative Example 2-8 (steel No. AJ), the addition of Ca was performed without taking into consideration the state where the Ca amount in the molten steel before the addition of Ca was high due to Ca as an impurity contained in the raw material of other elements added during secondary refining. For that reason, though the Ca amount feel within the scope of the present invention, the number of oxide-based non-metallic inclusions in steel comprised of Ca and Al and having a major diameter of 5 μm or more and satisfying the equation (1) was more than the upper limit of the scope of the present invention, the K_(ISSC) value was largely scattered, and one of the three specimens in the DCB test did not satisfy the target 26.4 MPa√m or more. 

The invention claimed is:
 1. A low alloy high strength seamless steel pipe for oil country tubular goods comprising a composition containing, in terms of mass %, C: 0.23 to 0.27%, Si: 0.01 to 0.35%, Mn: 0.45 to 0.70%, P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, Al: 0.015 to 0.080%, Cu: 0.02 to 0.09%, Cr: 0.8 to 1.5%, Mo: 0.5 to 1.0%, Nb: 0.02 to 0.05%, B: 0.0015 to 0.0030%, Ti: 0.005 to 0.020%, and N: 0.005% or less, and having a value of a ratio of the Ti content to the N content (Ti/N) of 3.0 to 4.0, with the balance being Fe and inevitable impurities, the steel pipe having a value (σ_(0.7)/σ_(0.4)), as a ratio of a stress at a strain of 0.7% to a stress at a strain of 0.4% in a stress-strain curve, of 1.02 or less and a yield strength of 655 MPa or more.
 2. The low alloy high strength seamless steel pipe for oil country tubular goods according to claim 1, which further contains, in addition to the composition, one or more selected from, in terms of mass %, V: 0.01 to 0.06%, W: 0.1 to 0.2%, and Zr: 0.005 to 0.03%.
 3. The low alloy high strength seamless steel pipe for oil country tubular goods according to claim 1, which further contains, in addition to the composition, in terms of mass %, Ca: 0.0005 to 0.0030%, and has the number of oxide-based non-metallic inclusions in steel comprised of Ca and Al and having a major diameter of 5 μm or more, whose composition ratio satisfies, in terms of mass %, the following equation (1), of 20 or less per 100 mm²: (CaO)/(Al₂O₃)≥4.0  (1).
 4. The low alloy high strength seamless steel pipe for oil country tubular goods according to claim 2, which further contains, in addition to the composition, in terms of mass %, Ca: 0.0005 to 0.0030%, and has the number of oxide-based non-metallic inclusions in steel comprised of Ca and Al and having a major diameter of 5 μm or more, whose composition ratio satisfies, in terms of mass %, the following equation (1), of 20 or less per 100 mm²: (CaO)/(Al₂O₃)≥4.0  (1). 